Hello, I just joined so i can ask this question. My wife drives a 2002 OBW, 2.5L, auto trans., with 180,000 mi. She bought it new and has been a great, relatively trouble free car. After much reading on this forum I decided to test the TPS due to a hesitation issue. I ended up disconnecting the plug from the TPS with the ignition switch on (not running) and heard a loud buzzing, like a solenoid not fully engaging, from the transmission. I quickly plugged it back in however, when I open the throttle past idle the buzzing starts. The car runs and does not make the sound while it is running.

I do all my own work and usually don't make dumb mistakes like disconnecting sensors with the ignition on.

Some history on the car. In the past year i have replaced the head gaskets, timing belt (every 60,000), wires and plugs and o2 sensors.
The TPS is shot, voltage drops at two points.

I had a similar issue. The O2 sensor buzzed. I replaced the O2 sensor, it still buzzed. I replaced the TPS and the MAF sensor. Its running fine, 1 year now, still on the Bosch O2 sensor.
here's the link

Thanks for the responses. No CEL or transmission light. After reading the thread on buzzing o2 sensor, i think that may be what is causing this. Exact same symptoms. I thought it was the transmission because when I felt the pan it was vibrating and the sound seems to emanate from that area.
Going to crawl under there after work and check.

With my (fly-by-wire) 07, if the key is On but the engine not started, and the gear selector is in a forward or reverse gear, there's a noticeable buzzing or rapid clicking from the transmission. It's coming from a duty solenoid (probably the line pressure solenoid) which functions by switching on and off rapidly, with the times on or off varying to control the pressure. With the engine off, the solenoid runs dry at a default rate, and so can be quite noisy.

All the duty solenoids other than the one for the AWD are located in the valve body of the transmission, which is at the bottom of the transmission, right where the oil pan is located.

There were posts here asking about similar clicking from the transmission on earlier models and as far as I know it seems to be normal.

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